A technique for eliminating the necessity of a dedicated auto-focus (AF) sensor and for achieving high-speed phase difference AF (so-called imaging plane phase difference AF) is known. The foregoing can be accomplished by providing an image element with a phase difference detecting function and using an image element including an imaging pixel group, respectively.
For example, according to PTL 1 (patent literature document 1), in some light-receiving elements (pixels) of an imaging element (sensor), a pupil-dividing function is provided by causing a sensitivity region of a light-receiving part to be arranged off-axis (not-concentric) relative to the optical axis of an on-chip microlens. According to this technique, pixels that receive a pair of pupil-divided light fluxes are referred to as an A-image pixel and a B-image pixel. The A-image pixel and the B-image pixel are defined as focus detection pixels. A plurality of A-image pixels and B-image pixels are arranged at specific intervals in an imaging pixel group, and phase-difference focus detection is performed on the basis of the amount of deviation between two images acquired from the A-image pixel group and the B-image pixel group.
Here, a focus detection pixel differs from a pixel dedicated to imaging in terms of light-receiving characteristics in that the light-receiving area of the focus detection pixel is smaller than the light-receiving area of the pixel dedicated to imaging or the center of gravity of a light-receiving region is off-axis (located elsewhere than at the geometrical center) relative to the optical axis of an on-chip microlens. Thus, since a defect occurs in part of image information at a position where a focus detection pixel is arranged, interpolation arithmetic operation is performed, on the basis of information of pixels dedicated to imaging on the periphery of the position where the focus detection pixel is arranged, to generate an image signal.
Here, when the arrangement density of focus detection pixels is set to low, the degradation in the image quality caused by the pixel defect can be reduced. However, the sampling characteristics of a focus detection image is deteriorated, and the focus detection performance for an object having a high spatial frequency (for example, an object is a small point source of light) is degraded. Furthermore, in the case where the arrangement density of focus detection pixels is high, the position of an A-image pixel cannot be made to completely match the position of a B-image pixel on the sensor plane. Thus, an object that can be complemented at an A-image pixel cannot be complemented at a B-image pixel. Therefore, by calculating the amount of image deviation for different objects between an A image and a B image, an erroneous range finding may occur.
In a technique disclosed in PTL 2, a determination as to whether or not an object is suitable for focus detection using a focus detection pixel is performed, on the basis of spatial frequency distribution of image data. By determining whether or not focus detection using a focus detection pixel is to be performed on the basis of a determination result, the influence of erroneous range finding caused by the deterioration of sampling characteristics can be reduced.